This invention relates to an electronic circuit which sets the value of a bit to one during several seconds after the power supply was removed from the said circuit, and which sets to zero the value of the said bit afterwards.
Electronic systems such as passive sensors for radio-frequency identification (RFID) are powered, for a short period of time, during which they fulfill their basic function of transmitting certain information to the reading terminal. After the power was removed, they are required to remember that a reading action just took place, and, for a duration of a few seconds, to not respond to any subsequent reading attempt. After this duration has elapsed, the sensor must respond to a new reading inquiry. This feature is achieved by including in the sensor certain circuitry which sets a bit to one for the few required seconds, and to zero afterwards.
A conventional technique uses the discharge of a capacitor through a resistor. The required time constant must be well controlled and in the range of seconds. However, this technique either requires a prohibitively large on-chip resistor, or an external capacitor, both solutions being unattractive for very low-cost sensors. Another conventional technique is to use the discharge of an on-chip capacitor through a low-leaking device such as a reverse-biased p-n junction or a MOS transistor. A drawback of this technique is that the leaking current of the discharging device highly varies with the temperature, typically exhibiting an exponential behavior. Therefore, the discharging time constant will have an inadmissible high temperature spread. To avoid the large temperature variation of the leakage current, another conventional technique adds a second current, having a tighter temperature variation, to the leakage current. The drawback consists in a higher discharging current than the leakage current, leading to larger capacitors.
It is therefore useful and desirable to use a circuit which outputs a one-bit signal, the said output signal being set and held to one for a duration of a few seconds after the power was removed from the circuit and subsequently re-applied, and being set and held to zero afterwards, wherein the duration when the said signal is held to one has a low variation with the ambient temperature and the process spread (the “persistence duration”), and which does not require large capacitors.